A circularly polarizing plate has a function of extracting circularly polarized light from non-polarized light. The circularly polarizing plate usually comprises a linearly polarizing membrane and a quarter wave (λ/4) plate. The linearly polarizing membrane has a function of extracting linearly polarized light from non-polarized light, and the quarter wave plate has a function of converting the extracted linearly polarized light to circularly polarized light.
The circularly polarizing plate has been used in various optical devices such as a reflective liquid crystal display, a pick up device for recording an optical disc, an organic electroluminescence (EL) display or an antireflection film. The optical characteristic of the circularly polarizing plate depends on the function of the quarter wave plate.
Various quarter wave plates have been proposed and used in optical devices. However, most of the conventional plates can function as a quarter-wave plate merely at a specific wavelength. If a quarter wave plate functions merely at a specific wavelength, a circularly polarizing plate using the quarter wave plate should function merely at the specific wavelength.
Japanese Patent Provisional Publication No. 5(1993)-27118 discloses a quarter wave plate comprising two polymer films, in which an angle between the slow axes of the films is essentially 90°. The disclosed quarter wave plate can function within a wide wavelength range.
Each of Japanese Patent Provisional Publication Nos. 10(1998)-68816 and 10(1998)-90521 discloses a quarter wave plate comprising two polymer films, in which an angle between the slow axes of the films is essentially 60°. The disclosed quarter wave plate can also function within a wide wavelength range.